The present invention generally relates to optical semiconductor devices and more particularly to fabrication of a laser diode having a diffraction grating.
Laser diodes are used extensively in the field of optical information processing including optical telecommunication.
In the field of optical telecommunication, various optical modulation methods are proposed so far for transmitting as much information as possible via a single optical fiber. Among others, the optical wavelength multiplexing method, in which a number of optical signals having respective, various wavelengths are transmitted through an optical fiber in the form of a wavelength multiplexed optical signal, is a promising process for increasing the amount of information transmitted through the optical fiber. In order to realize such an optical wavelength multiplex system, it is necessary to provide a number of laser diodes that is capable of oscillating at respective, mutually different wavelengths with stability and reliability.
A laser diode generally includes an optical cavity and an active layer in which stimulated emission of photons occurs. In the laser diodes for use in the foregoing wavelength multiplexing system, however, it is preferable and advantageous to use a diffraction grating in place of the optical cavity, as in the case of a DFB (distribution feedback) laser diode, so as to be able to set the oscillation wavelength of the laser diode to a desired wavelength. Further, the use of a DBR laser diode, in which the reflectors of the optical cavity are replaced by a diffraction grating, is also possible. By using such a diffraction grating, it is possible to change the wavelength of the optical beam that causes a resonance with the diffraction grating easily by changing the pitch of the diffraction grating. In other words, it is possible to fabricate the laser diodes easily to have respective oscillation wavelengths so as to be coincident to the desired wavelengths of the optical signal components, by setting the diffraction grating pitch in accordance with the desired optical wavelengths.
FIGS. 1A-1C show the construction of a DFB laser diode proposed in the U.S. Pat. No. 5,170,402, wherein FIGS. 1A and 1C show the laser diode in a longitudinal cross-sectional view and in a lateral cross-sectional view respectively, while FIG. 1C shows a photon density profile in the laser diode along a longitudinal direction thereof.
Referring to FIGS. 1A and 1C, the laser diode is constructed on a substrate 1 of n-type InP on which a diffraction grating 1A is formed such that the diffraction grating 1A extends in the longitudinal direction of the laser diode.
On the diffraction grating 1A, a waveguide layer 2 of n-type InGaAsP is provided, and an active layer 3 having an MQW (multiple quantum well) structure is provided on the waveguide layer 2. It should be noted that the MQW structure of the active layer 3 includes an alternate repetition of a quantum well layer of undoped GaInAs and an undoped barrier layer of GaInAsP, wherein the quantum well layer has a thickness below a de Broglie wavelength of the carriers in the quantum well layer. By reducing the thickness of the quantum well layer as such, quantum levels are formed in the quantum well layer.
On the active layer 3, a cladding layer 4 of p-type InGaAsP and a cap layer of P.sup.+ -type InGaAsP are provided successively, and ohmic electrodes 6A, 6B and 6C are provided on the cap layer 5 such that the electrodes 6A, 6B and 6C are aligned in the longitudinal direction of the laser diode. Further, a lower electrode 7 is provided on the lower major surface of the substrate 1 in ohmic contact therewith.
In the illustrated example, the laser diode includes a .LAMBDA./2 phase-shift point 1B at a mid point of the diffraction grating 1A in which the pitch of the diffraction grating 1A is shifted in the longitudinal direction of the laser diode by .LAMBDA./2, wherein .LAMBDA. represents the pitch of the diffraction grating 1A. In terms of the optical beam that is diffracted in the waveguide layer 2 by the diffraction grating 1A, the foregoing .LAMBDA./2 phase-shift point 1B induces an optical phase shift of .lambda./4 in the optical beam, wherein .lambda. represents the wavelength of the optical beam. By forming the .LAMBDA./2 phase shift point 1B as such, the photon density in the active layer 3 becomes maximum in correspondence to the phase shift point 1B provided at the longitudinally midpoint of the laser diode as indicated in FIG. 1B. It should be noted that such a maximum of the photon density in turn induces a minimum of the carrier density due to the facilitated stimulated emission caused by the maximum photon emission.
Thus, it becomes possible, in the laser diode of FIGS. 1A-1C, to modulate the carrier density profile and hence the refraction index of the laser diode effectively by injecting a signal current via the electrode 6B provided in correspondence to such a minimum of the carrier distribution profile, while simultaneously driving the laser diode by injecting driving currents via the electrodes 6A and 6C. In other words, the laser diode functions as a tunable laser diode.
When constructing a wavelength multiplex telecommunication system by using such a laser diode, it is necessary to fabricate a number of tunable laser diodes such that the laser diodes have respective central wavelengths different from each other. For this purpose, there is a need for a technology that enables formation of the devices having the respective diffraction gratings with mutually different grating pitches.
While it is not particularly difficult to change the diffraction grating pitch device by device, the laser diodes for use in a wavelength multiplex telecommunication system are desired to be provided in the form of optical integrated circuit in which the laser diodes having respective, different wavelengths are formed on a common substrate. In such a case, it is necessary to form a number of such diffraction gratings on a common substrate with mutually different pitches.
Conventionally, the diffraction grating of a DFB laser diode or a DBR laser diode has been formed by a dual-beam interference exposure process that uses interference fringes formed as a result of interference of two optical beams.
FIG. 2 shows the principle of forming the diffraction grating by such a dual-beam interference exposure process.
Referring to FIG. 2, an optical beam having a wavelength .lambda. produced by a coherent source such as a He-Cd laser is split into a first optical beam and a second optical beam, and the first and second optical beams thus split are directed to a substrate on which the diffraction grating is to be formed, with respective incident angles .THETA..sub.1 and .THETA..sub.2. The substrate is covered by a photoresist film, and the photoresist film is exposed according to a desired diffraction grating pattern with a pitch .LAMBDA..sub.1, which is given as EQU .LAMBDA..sub.1 /.LAMBDA./(sin .THETA..sub.1 +sin .THETA..sub.2).
In the exposure process of FIG. 2, it should be noted that the incident angles .THETA..sub.1 and .THETA..sub.2 are converted to incident angles .THETA..sub.3 and .THETA..sub.4 respectively by disposing a prism as indicated in FIG. 3, wherein the incident angles .THETA..sub.3 and .THETA..sub.4 are given according to the relationship EQU .THETA..sub.3 =sin.sup.-1 [n.times.sin{sin.sup.-1 (sin{(.THETA..sub.1 +.phi.)/n}-.phi.}] EQU .THETA..sub.4 =sin.sup.-1 [n.times.sin{sin.sup.-1 (sin{(.THETA..sub.2 -.phi.)/n}+.phi.}]
where .phi. represents a slope angle of the inclined surface of the prism shown in FIG. 3, while n represents the refractive index of the prism.
When such a prism is used, the exposed diffraction pattern now has a pitch .sub.-2 given according to the relationship EQU .LAMBDA..sub.2 =.lambda./(sin .THETA..sub.3 +sin .THETA..sub.4).
Thus, the inventor of the present invention has previously proposed, in the Japanese Laid-Open Patent Publication 63-341879, an exposure process of a diffraction grating that uses a prism shown in FIG. 4 in the dual-beam interference exposure process of FIG. 4, such that the pitch of the exposed diffraction pattern changes in a first area on the substrate and in a second area of the substrate. A similar proposal is made in the Japanese Laid-Open Patent Publication 6-97600.
FIG. 5A shows a prism 10 that is proposed in the Japanese Laid-Open Patent Publication 63-97600, op. cit., while FIG. 5B shows an example of the diffraction pattern exposed by using the prism 10 of FIG. 5A.
Referring to FIG. 5A, the prism 10 includes a plurality of regions 10A-10D having respective, different inclination angles for the sloped surface, and thus, the substrate 11 is formed with diffraction gratings 11A-11D with respective pitches in correspondence to the foregoing prism regions 10A-10D.
In the construction of FIG. 4 or FIGS. 5A and 5B, it should be noted that there is formed a step between adjacent sloped surfaces of the prism. As such a step causes a diffraction in the optical beam passing through the prism, the diffraction grating pattern exposed on the substrate is substantially distorted from the ideal pattern shown in FIG. 5B as a result of the diffraction of the optical beam thus caused. This problem is particularly serious in the laser diode array for use in wavelength multiplexed optical telecommunication systems, in which the laser diodes are arranged on a common substrate adjacent with each other with a minute interval or mutual separation. Further, it should be noted that the prism 10 of FIG. 5A is preferably manufactured such that each of the regions 10A-10D has a width of about 300 .mu.m or less. However, precise formation of such a small prism surface is difficult.
It is of course possible to form a diffraction grating pattern on a substrate with a varying grating pitch, by using an electron beam exposure system and process. However, such an electron beam exposure of the diffraction grating pattern needs enormously long time, as the grating pattern has to be exposed one line by one line by using a single electron beam. Further, the diffraction grating pattern formed by an electron beam exposure process tends to show a fluctuation of grating pitch due to a voltage fluctuation.